Process for manufacturing galvannealed steel sheets having high press-formability and anti-powdering property

ABSTRACT

It is an object to provide a process for manufacturing galvannealed steel sheets having high anti-powdering property as required when they are press formed, and stabilized frictional properties in a coil. Steel sheets are plated in a bath having a low aluminum content, while they have, when entering the bath, a low temperature as defined in relation to the aluminum content of the bath, so that an alloying reaction may be prevented. Then, the sheets are heated for alloying in a high-frequency induction heating furnace so that the sheets leaving the furnace may have a temperature of from over 495° C. to 520° C. to yield galvannealed consisting mainly of a δ 1  phase. An iron or iron-alloy top coating having an appropriate iron content can be applied onto the plated steel surface to improve its paintability.

TECHNICAL FIELD

This invention relates to a process for manufacturing galvannealed steelsheets which are used for making automobile bodies and parts, etc., andparticularly which exhibit excellent anti-powdering property when pressformed, and stable frictional properties in a coil.

BACKGROUND ART

There has recently been an increasing demand for galvannealed steelsheets as the rust-proof steel sheets for automobiles, since theyexhibit high corrosion resistance and weldability when painted. Thelatest tendency has been toward sheets having a heavier c/w to ensurehigh corrosion resistance.

These galvanized steel sheets are required to have excellentpress-formability and exhibit excellent anti-powdering property whenpress formed. These requirements have lately been becoming morestringent, and the increasing coating weight has been creating a bigproblem in the maintenance of, above all, excellent anti-powderingproperty.

There is known a process which comprises heating galvanized steel sheetsrapidly to cause the alloying of a part of coating, and batch annealingthem to improve their anti-powdering property. This process is effectivein achieving an improved anti-powdering property, but has the drawbackof being expensive.

Japanese Laid-Open Patent Application No. Hei 1-279738 discloses aprocess for achieving an improved anti-powdering property in line.According to its disclosure, steel sheets are plated in a bathcontaining 0.04 to 0.12% A1, are heated to a temperature of at least470° C. rapidly within two seconds to undergo alloying, and are cooledto a temperature not exceeding 420° C. rapidly within two seconds,whereby galvannealed steel sheets consisting mainly of a δ₁ phase aremanufactured.

Due to the relatively high temperature which the process employs for thealloying treatment, however, it is very likely that alloying may proceedso rapidly that the growth of a thick Γ phase may result in a lowanti-powdering property. Although Japanese Laid-Open Patent ApplicationNo. Hei 1-279738 proposes rapid cooling within two seconds from thetemperature range in which alloying is effected, to the temperaturerange not exceeding 420° C. to prevent excessive alloying, a properalloying pattern varies with the coating weight and the line speed, andthe use of the process, therefore, calls for the provision of amultiplicity of sources of heating and cooling mediums along a line andthereby brings about an increase in the cost of equipment.

Moreover, a direct gas-fired alloying furnace which is usually employedis likely to have a temperature variation along the width and length ofa steel strip, and such temperature variation makes difficult the strictcontrol of the coating structure as hereinabove stated and results inthe formation of a coating having excessively alloyed portions orcontaining a residual ζ phase. The resulting plated steel sheet lacksuniformity in the amount of its δ₁ phase and therefore in itsanti-powdering property. The amount of the ζ phase has so close abearing on the frictional properties that those portions of the platedsteel sheet which contain the residual ζ phase have a higher frictionalcoefficient and are, therefore, lower in press formability.

DISCLOSURE OF THE INVENTION

In view of the problems of the prior art as hereinabove pointed out, we,the inventors of this invention, have studied an alloying reaction on agalvanized steel sheet, and found the following:

(1) The ζ phase is formed by a reaction at or below 495° C., and is notformed at any temperature exceeding it; and

(2) Therefore, it is possible to form a coating consisting mainly of aδ₁ phase if the principal reaction (the reaction which causes a moltenzinc phase to disappear) is caused to take place at a temperatureexceeding 495° C., followed by cooling. FIGS. 1 and 2 show by way ofexample phase changes resulting from isothermal alloying reactions ongalvanized steel sheets at 450° C. and 500° C., respectively. While thealloying at 450° C. results in the formation of a ζ phase, the alloyingat 500° C. hardly brings about any ζ phase, but forms a coatingconsisting mainly of a δ₁ phase.

The use of such a relatively high temperature for alloying is, however,likely to result in an excessively alloyed coating which is low inanti-powdering property, as hereinabove stated. Moreover, a usualdirect-fired alloying furnace is difficult to employ to achievecombustion which is uniform from the standpoints of both time and place,and is likely to cause uneven firing. Such uneven firing forms an alloylayer lacking uniformity, and results only in a product lackinguniformity in anti-powdering property, frictional properties, etc.depending upon the portions of the steel strip.

Under these circumstances, we have tried to explore a process which canalways reliably be employed to achieve both anti-powdering property andpress formability which are satisfactorily excellent, and havediscovered the following:

(1) It is possible to obtain a coating in which an alloy layerconsisting mainly of a δ₁ phase is formed uniformly along the width andlength of a strip, if any alloying reaction (formation of a ζ phase) ina zinc bath is inhibited, and if the subsequent alloying treatment iscarried out by employing a high-frequency induction heating furnace;

(2) The resulting alloyed coating exhibits excellent anti-powderingproperty and press formability owing to the alloying reaction takingplace uniformly not only macroscopically as hereinabove stated, but alsomicroscopically;

(3) It is possible to achieve a strict coating control if the conditionsof the bath and the temperature of the strip leaving the high-frequencyinduction heating furnace are appropriately specified, or morespecifically, it is possible to control the alloying reaction (formationof a ζ phase) in the bath adequately if the bath has a low aluminumcontent and if the strip entering the bath has a relatively lowtemperature as defined in relation to the aluminum content of the bath,and it is possible to obtain the coating as described at (1) and (2)above if the alloying treatment for the galvanized strip in thehigh-frequency induction heating furnace is so performed that the stripleaving the furnace may have a temperature of from over 495° C. to 520°C.; and

(4) The alloyed coating exhibits good paintability at a small coatingweight if it is covered with an iron or iron-alloy top coating.

This invention is based on the foregoing discovery, and consistsessentially in:

[1] A process for manufacturing galvannealed steel sheets havingexcellent press-formability and anti-powdering property by galvanizing asteel strip in a zinc bath containing aluminum, the balance of itscomposition being zinc and unavoidable impurities, controlling itscoating weight, and subjecting the strip to alloying treatment in aheating furnace so that its coating may have an iron content of 8 to12%, characterized in that the bath has an aluminum content of at least0.05%, but less than 0.13%, and a temperature not exceeding 460° C.,that the strip has, when entering the bath, a temperature satisfying thefollowing relationship:

    437.5×[Al%]+428>T≧437.5×[Al%]+408

where

[Al%]: the aluminum content (%) of the bath;

T: the temperature (°C.) of the strip entering the bath, so that anyreaction causing the alloying of iron and zinc may be prevented fromoccurring in the bath, and that the furnace is a high-frequencyinduction furnace in which the strip is heated so as to have atemperature of from over 495° C. to 520° C. when leaving the furnace,the strip being held at that temperature for a predetermined length oftime, and cooled; and

[2] A process for manufacturing galvannealed steel sheets havingexcellent press-formability and anti-powdering property by galvanizing asteel strip in a zinc bath containing aluminum, the balance of itscomposition being zinc and unavoidable impurities, controlling itscoating weight, and subjecting the strip to alloying treatment in aheating furnace so that its coating may have an iron content of 8 to12%, characterized in that the bath has an aluminum content of at least0.05%, but less than 0.13%, and a temperature not exceeding 460° C.,that the strip has, when entering the bath, a temperature satisfying thefollowing relationship:

    437.5×[Al%]+428>T≧437.5×[Al%]+408

where

[Al%]: the aluminum content (%) of the bath;

T: the temperature (°C.) of the strip entering the bath, so that anyreaction causing the alloying of iron and zinc may be prevented fromoccurring in the bath, and that the furnace is a high-frequencyinduction furnace in which the strip is heated so as to have atemperature of from over 495° C. to 520° C. when leaving the furnace,the strip being held at that temperature for a predetermined length oftime, and cooled, and that the strip is plated with a top coating havingan iron content of at least 50% and coating weight of at least 2 g/m².

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows by way of example the phase changes occurring ingalvannealed steel sheets as a result of the isothermal reaction at 450°C.

FIG. 2 shows by way of example the phase changes occurring ingalvannealed steel sheets as a result of the isothermal alloyingreaction at 500° C.

DETAILED DESCRIPTION OF THE INVENTION

The alloying treatment of galvanized steel sheets by high-frequencyinduction heating is known, as described in, for example, JapanesePatent Publication No. Sho 60-289 and Japanese Laid-Open PatentApplication No. Hei 2-37425. The arts disclosed therein are, however,nothing but the use of high-frequency induction heating as a means forrapid heating.

On the other hand, this invention is based on the discovery of the factthat, if the alloying reaction in the bath is inhibited as far aspossible, and if the coating in which alloying has been inhibited issubjected to alloying treatment by high-frequency induction heatingunder specific conditions, it is possible to form an alloy layer hardlyhaving any Γ phase, but consisting mainly of a δ₁ phase uniformly on asteel strip and produce a plated steel strip having an overallyexcellent anti-powdering property due to the microscopic uniformity ofits coating structure, as well as high press-formability.

It is presumably for the reasons as will hereunder be set forth that theprocess of this invention can manufacture plated steel sheets havingoutstanding properties as hereinabove stated.

In the first place, the use of high-frequency induction heating for thealloying treatment enables the direct heating of the strip andparticularly of its surface contacting the coating which, as opposed togas heating, allows the reaction of iron and zinc to occur rapidly anduniformly on the surface of any strip portion and thereby form a layernot having any excessively alloyed portion or any residual ζ phase, butexhibiting uniform anti-powdering property and press formability.

In the second place, the direct heating of the strip as hereinabovestated apparently brings about an even microscopically uniform alloyingreaction. The conventional alloying treatment by gas heating is likelyto lack heating uniformity and result in an alloying reaction whichmicroscopically lacks uniformity, since heat is applied from the outsideof the coating. The grain boundary is particularly high in reactivityand is, therefore, likely to undergo the so-called outburst reactionforming an outburst structure which causes the growth of a Γ phaselowering the anti-powdering property of the coating. On the other hand,high-frequency induction heating, which enables the direct heating ofthe strip, enables a substantially uniform alloying reaction andfacilitates the diffusion of oxides on the strip and an alloyinginhibitor (Fe₂ Al₅) formed in the bath, thereby enabling the formationof an even microscopically uniform alloy layer.

In the third place, high-frequency induction heating allows only alimited length of time for the growth of the Γ phase, as it enables therapid alloying of the coating. This invention can greatly restrict theoverall formation of the Γ phase, as it also inhibits the formation ofthe Γ phase in the bath. This apparently contributes greatly toachieving an improved anti-powdering property.

In the fourth place, high-frequency induction heating has the advantageof enabling the uniform heating of the strip along its width and length,and thereby the strict control of the temperature of the strip leavingthe heating furnace. Moreover, there can hardly occur any excessivealloying even without any special cooling, since there is no heated andrising atmosphere gas (due to the draft effect) as in any heatingapparatus employing an atmosphere gas, such as a gas-fired furnace.

Description will now be made of the essential features of this inventionand the reasons for the limitations employed to define it.

According to this invention, the aluminum content of a zinc bath, thetemperature of a steel strip entering the bath and the bath temperatureare so specified as to prevent any alloying reaction in the bath as faras possible. According to one of the salient features of this invention,the bath has a low aluminum content and the strip entering the bath hasa relatively low temperature as defined in relation to the aluminumcontent of the bath, so that any alloying reaction in the bath may beprevented.

While it is necessary to plate in a bath having a low aluminum content astrip having a low temperature when entering the bath in order toprevent any alloying reaction (formation of a ζ phase) in the bath, Fe₂Al₅ does not effectively prevent alloying in any bath having an aluminumcontent of less than 0.05%, but an outburst reaction takes place in thebath and brings about a inferior anti-powdering property. Therefore, itis necessary for the bath to have an aluminum content of at least 0.05%.If the bath has an aluminum content of 0.13% or more, however, theexcessive inhibition of the-alloying reaction of iron and zinc in thebath calls for an abrupt alloying reaction during the subsequentalloying treatment and such an abrupt alloying reaction brings about aninferior anti-powdering property. Therefore, it is necessary for thebath to have an aluminum content of less than 0.13%.

The temperature of the strip entering the bath is required to satisfythe following relationship to the aluminum content of the bath:

    437.5×[Al%]+428>T≧437.5×[Al%]+408

where

[Al%]:the aluminum content (%) of the bath;

T:the temperature (°C.) of the strip entering the bath.

If the temperature of the strip entering the bath exceeds the upperlimit of the range as defined above, the alloying reaction takes placein the bath and forms a ζ phase, thereby disabling the formation of analloy layer consisting mainly of a δ₁ phase as intended by thisinvention. If the temperature is lower than the lower limit, theformation of Fe₂ Al₅ in a way lacking uniformity brings about a localalloying reaction resulting in a lower anti-powdering property.

The bath is required to have a temperature not exceeding 460° C., sincea higher temperature promotes an alloying reaction in the bath.Moreover, too high a bath temperature brings about problems includingthe formation of dross by the erosion of structural members immersed inthe bath.

The strip which has been galvanized is heated for alloying in ahigh-frequency induction heating furnace. The heating by ahigh-frequency induction heating furnace is a salient feature of thisinvention other than the bath conditions as hereinabove set forth, sinceno alloyed coating as intended by this invention can be obtained by theconventional gas heating as hereinbefore stated. The allowing treatmentis carried out by heating the strip so that the strip leaving thefurnace may have a temperature of from over 495° C. to 520° C., holdingit for a predetermined length of time, and cooling it. Heating at atemperature exceeding 495° C. is necessary to form a δ₁ phase, ashereinabove stated, so that the coating which has been prevented fromalloying in the bath is alloyed to form an alloy layer consisting mainlyof a δ₁ phase. The heating temperature has, however, an upper limit of520° C., since heating at a temperature exceeding 520° C. forms a Γphase resulting in a inferior anti-powdering property. The striptemperature is controlled at the exit of the high-frequency inductionheating furnace, since in that area, the strip reaches the maximumtemperature in an alloying heat cycle. The control of the striptemperature at the exit of the furnace enables an alloying reaction atthat temperature, since the rate of growth of the alloy layer reachesthe maximum in that area.

This invention is intended for manufacturing galvannealed steel sheetshaving a coating containing 8 to 12% of iron. A coating containing morethan 12% of iron is hard, and low in anti-powdering property. Ifalloying is continued beyond the exit of the high-frequency inductionheating furnace, a diffusion reaction in a solid results in theformation of a coating having a higher iron content. A coating having aniron content of less than 8% is undesirable, since an η phase (purezinc) remains on the surface of the coating and causes flaking when thestrip is press formed.

Although it has hitherto been believed that the iron content of acoating has a decisive bearing on its structure, the appropriatelyselected bath conditions and the alloying treatment by high-frequencyinduction heating, as proposed by this invention, enable the formationof a specific coating structure as intended by this invention,irrespective of its iron content.

The alloyed coating obtained as hereinabove described is composed of auniform δ₁ phase on its surface and a very thin Γ phase underlying it.

An iron or iron-alloy top coating having an iron content of at least 50%and a coating weight of at least 2 g/m² can be applied onto the alloyedcoating to improve its paintability. A galvannealed steel sheet islikely to develop during electrodeposition a defect called crateringwhich exerts an adverse effect on the appearance of a finally paintedsurface. The top coating prevents the occurrence of any such paintingdefect and improves the paintability of the sheet. The top coatingpreferably consists solely of an α phase to ensure improvedpaintability. An iron or iron-alloy coating having an iron content of atleast about 50% consists solely of an α phase.

No top coating weight that is less than 2 g/m² is satisfactory forimproving paintability. Although the top coating weight has noparticular upper limit, it is preferable from an economical standpointto set an upper limit of 5 g/m². The high-frequency induction heating ofthe galvanized strip which is followed by electroplating the top coatingtherefor as proposed by this invention, does not cause any oxidation ofthe coating surface, but enables the appropriate application of the topcoating onto the alloyed coating surface, and thereby a reduction in topcoating weight, as compared with what is required on a coating alloyedby gas heating.

EXAMPLES

Examples of this invention are shown in TABLES 1 to 4.

These examples were carried out by employing as starting materials coldrolled sheets of Al-killed steel (containing 0.03% C and 0.02% sol. Al)and Ti-containing IF steel (containing 0.0025% C, 0.04% sol. Al and0.07% Ti), and galvanizing them, heating them and top coating a part ofthem, under the conditions shown in TABLES 1 and 2. The heating was gasor high-frequency induction heating. The anti-powdering property, pressformability, and paintability of the galvannealed steel sheets whichwere obtained are shown in TABLES 3 and 4.

The temperature of the sheet entering the plating bath was its surfacetemperature as measured by a radiation pyrometer immediately before itentered the bath. The temperature of the sheet leaving the heatingfurnace was its surface temperature as measured by a radiation pyrometerat the discharge end of the furnace.

The aluminum content of the bath is the effective aluminum concentrationas defined by the following equation:

    [Effective Al concentration]=[Total Al concentration of bath]-[Iron concentration of bath]+0.03

The percentage of iron in the coating depends on the bath conditions,and the heating and cooling conditions. The cooling conditions vary thedegree of alloying (% of Fe in the coating) and thereby affect itsproperties, though they hardly have any effect on the macroscopic ormicroscopic uniformity of the coating structure defining one of thesalient features of this invention. Therefore, the examples were carriedout by controlling the capacity of a cooling blower and the amount ofmist to regulate the percentage of iron in the coating.

The following is a description of the methods which were employed fortesting and evaluating the products for properties:

Amount of ζ phase in coatings on products:

The peak intensity, I ζ.sub.[ 421], of the ζ phase at d=1.900 and thepeak intensity, I δ₁[249], of the δ₁ phase at d=1.990 were determined bythe X-ray diffraction of the coating, and their ratio was calculated inaccordance with the following equation as representing the amount of theζ phase in the coating. I_(BG) represents the background, and if Z/Ddoes not exceed 20, there is substantially no ζ phase.

    Z/D=(I ζ.sub.[421] -I.sub.BG)/(I δ.sub.1[249] -I.sub.BG)×100

Anti-powdering property

After each specimen had been coated with 1 g/m² of a rust-preventing oil(Nox Rust 530F of Parker Industries, Inc.), a draw bead test wasconducted by employing a bead radius R of 0.5 mm, a holding load P of500 kg and an indentation depth h of 4 mm. After a tape had been peeledoff, the amount of powdering was calculated from a difference in weightof the specimen from its initial weight. Each of the values appearing inthe tables is the average of a plurality of values as measured (5×5=25).

Maximum deviation of anti-powdering property along strip width

The anti-powdering property of each strip was measured at five pointsalong its length and at five points along its width (both edges, midwaybetween each edge and the center, and the center) in a region havingstabilized operating conditions. The difference between the maximum andminimum values was taken as the maximum deviation.

Coefficient of friction

After each specimen had been coated with 1 g/m² of rust-preventing oil(Nox Rust 530F of Parker Industries, Inc.), an indenter made of toolsteel SKD 11 was held against the specimen under a load of 400 kg and itwas drawn at a speed of 1 m/min. The ratio of the drawing load and theholding load was taken as the frictional coefficient. Each of the valuesappearing in the tables is the average of a plurality of values asmeasured (5×5=25).

Maximum deviation of coefficient of friction along strip width

The coefficient of friction was measured at the same points as those atwhich the anti-powdering property had been measured, and the differencebetween the maximum and minimum values was taken as the maximumdeviation.

In Comparative Examples 1 and 2, the frictional properties were bad dueto the formation of a ζ phase in the bath, as the temperatures of thestrips entering the bath were too high. The product of ComparativeExample 3 was bad in anti-powdering property due to the microscopicallynon-uniform alloying as a result of the non-uniform formation of Fe₂ Al₅in the bath, as the temperature of the strip entering the bath was low.The product of Comparative Example 4 was bad in frictional propertiesdue to the formation of a ζ phase in the coating, as the temperatureachieved by high-frequency induction heating was too low. Theproducts-of. Comparative Examples 5 and 10 were bad in anti-powderingproperty due to the formation of a thick Γ phase, as the temperaturesachieved by high-frequency induction heating were too high.

Gas heating was employed in Comparative Examples 6 to 8. The product ofComparative Example 6, in which a relatively high temperature wasemployed, was bad in anti-powdering property due to the partialformation of a Γ phase as a result of uneven firing, and showedfrictional properties varying along the strip width. The products ofComparative Examples 7 and 8, in which lower temperatures were employed,were bad in both anti-powdering and frictional properties due to thepartially remaining ζ phase as a result of uneven firing, and showedgreatly varying results along the strip width.

Comparative Example 9 was carried out to enable comparison with respectto the top coating weight.

                                      TABLE 1                                     __________________________________________________________________________              Undercoat plating conditions                                                                         Temp.                                                  Temp. of               of strip    Fe content                                                                          Top *.sup.2 Amount                *.sup.1                                                                          strip Al content       leaving the                                                                          Coating                                                                            of the                                                                              coating                                                                           of ζ phase                                                               in                            Steel                                                                            entering                                                                            of bath                                                                             Line speed heating                                                                              weight                                                                             coating                                                                             weight                                                                            product                No.    type                                                                             bath (°C.)                                                                   (wt %)                                                                              (mpm) Heating                                                                            furnace (°C.)                                                                 (g/m.sup.2)                                                                        (wt %)                                                                              (g/m.sup.2)                                                                       (Z/D)                  __________________________________________________________________________    Invention's                                                                          A  468   0.117 100   Inducting                                                                          505    58.7 10.5  --  17.0 (none)            Example 1                   heating                                           Invention's                                                                          A  454   0.087 100   Inducting                                                                          507    59.2 10.2  --  16.2 (none)            Example 2                   heating                                           Invention's                                                                          A  453   0.066 100   Inducting                                                                          506    58.3 10.7  --  15.8 (none)            Example 3                   heating                                           Invention's                                                                          A  440   0.056 100   Inducting                                                                          505    58.9 10.5  --  16.7 (none)            Example 4                   heating                                           Comparative                                                                          A  470   0.070 100   Inducting                                                                          510    60.1 10.3  --  32.1                   Example 1                   heating                                           Comparative                                                                          A  482   0.100 100   Inducting                                                                          508    59.2 10.5  --  30.6                   Example 2                   heating                                           Comparative                                                                          A  450   0.120 100   Inducting                                                                          505    59.4 10.2  --  18.1 (none)            Example 3                   heating                                           Invention's                                                                          A  455   0.092  90   Inducting                                                                          515    65.2 9.8   --  19.3 (none)            Example 5                   heating                                           Invention's                                                                          A  457   0.094  90   Inducting                                                                          500    64.8 9.9   --  18.7 (none)            Example 6                   heating                                           Comparative                                                                          A  455   0.091  90   Inducting                                                                          490    64.2 10.0  --  28.5                   Example 4                   heating                                           Comparative                                                                          A  456   0.095  90   Inducting                                                                          525    65.3 9.8   --  18.4 (none)            Example 5                   heating                                           __________________________________________________________________________     *.sup.1 Steel type A: Alkilled steel; Steel type B: Ticontaining IF steel     *.sup.2 No ζ phase if Z/D is not more than 20                       

                                      TABLE 2                                     __________________________________________________________________________              Undercoat plating conditions                                                                         Temp.                                                  Temp. of               of strip    Fe content                                                                          Top *.sup.2 Amount                *.sup.1                                                                          strip Al content       leaving the                                                                          Coating                                                                            of the                                                                              coating                                                                           of ζ phase                                                               in                            Steel                                                                            entering                                                                            of bath                                                                             Line speed heating                                                                              weight                                                                             coating                                                                             weight                                                                            product                No.    type                                                                             bath (°C.)                                                                   (wt %)                                                                              (mpm) Heating                                                                            furnace (°C.)                                                                 (g/m.sup.2)                                                                        (wt %)                                                                              (g/m.sup.2)                                                                       (Z/D)                  __________________________________________________________________________    Invention's                                                                          B  474   0.122 100   Inducting                                                                          504    58.8 10.5  --  19.5 (none)            Example 7                   heating                                           Invention's                                                                          B  475   0.122 100   Inducting                                                                          508    57.8 10.6  --  18.9 (none)            Example 8                   heating                                           Invention's                                                                          B  474   0.120 100   Inducting                                                                          516    58.6 10.4  --  18.8 (none)            Example 9                   heating                                           Comparative                                                                          A  460   0.063 90    Gas  515    60.1 11.2  --  19.3 (none)            Example 6                   heating                                           Comparative                                                                          A  462   0.062 90    Gas  508    61.4 10.9  --  20.0                   Example 7                   heating                                           Comparative                                                                          A  460   0.064 90    Gas  500    60.5 10.6  --  22.6                   Example 8                   heating                                           Comparative                                                                          A  460   0.100 80    Inducting                                                                          505    55.5 9.8   0.8 17.2 (none)            Example 9                   heating                                           Invention's                                                                          A  461   0.100 80    Inducting                                                                          507    56.1 9.9   2.2 16.8 (none)            Example 10                  heating                                           Invention's                                                                          A  460   0.100 80    Inducting                                                                          506    55.9 9.8   3.5 17.0 (none)            Example 11                  heating                                           Invention's                                                                          A  478   0.121 80    Inducting                                                                          506    50.2 10.0  2.5 17.0 (none)            Example 12                  heating                                           Invention's                                                                          A  480   0.122 80    Inducting                                                                          515    52.1 10.3  2.7 17.2 (none)            Example 13                  heating                                           Comparative                                                                          A  478   0.121 80    Inducting                                                                          530    50.7 10.7  2.8 17.0 (none)            Example 10                  heating                                           __________________________________________________________________________     *.sup.1 Steel type A: Alkilled steel; Steel type B: Ticontaining IF steel     *.sup.2 No ζ phase if Z/D is not more than 20                       

                                      TABLE 3                                     __________________________________________________________________________                *.sup.1 Anti-                                                                       *.sup.2 Maximum                                                                             *.sup.4 Maximum                                           powdering                                                                           deviation along                                                                             deviation                                                 property                                                                            strip width                                                                           *.sup.3 Frictional                                                                  along                                         No.         (g/m.sup.2)                                                                         (g/m.sup.2)                                                                           coefficient                                                                         strip width                                                                         Remarks                                 __________________________________________________________________________    Invention's Example 1                                                                     4.3   0.22    0.135 0.002                                         Invention's Example 2                                                                     4.0   0.20    0.133 0.002                                         Invention's Example 3                                                                     4.5   0.26    0.136 0.002                                         Invention's Example 4                                                                     4.5   0.23    0.139 0.003                                         Comparative Example 1                                                                     4.6   0.27    0.148 0.002 ζ phase formed in the bath.        Comparative Example 2                                                                     4.7   0.28    0.145 0.003 ζ phase formed in the bath.        Comparative Example 3                                                                     7.2   0.42    0.136 0.003 The non-uniform formation of                                                  Fe.sub.2 Al.sub.5 brought about                                               microscopic non-uniformity and low                                            anti-powdering                                                                property.                               Invention's Example 5                                                                     5.8   0.28    0.138 0.002                                         Invention's Example 6                                                                     5.6   0.27    0.136 0.003                                         Comparative Example 4                                                                     4.9   0.25    0.145 0.006 Because of the strip leaving                                                  temperature of high                                                           frequency induction heating farnace                                           is high, ζ                                                               phase is formed.                        Comparative Example 5                                                                     8.4   0.44    0.133 0.003 Because of the strip leaving                                                  temperature of high                                                           frequency induction heating farnace                                           is high, anti-                                                                powdering property is                   __________________________________________________________________________                                          low.                                     *.sup.1 Good if it is not more than 6 g/m.sup.2 (at a coating weight of 6     g/m.sup.2)                                                                    *.sup.2 Good if it is not more than 0.3 g/m.sup.2                             *.sup.3 Good if it is not more than 0.14                                      *.sup.4 Good if it is not more than 0.003                                

                                      TABLE 4                                     __________________________________________________________________________                *.sup.1 Anti-                                                                       *.sup.2 Maximum                                                                             *.sup.4 Maximum                                           powdering                                                                           deviation along                                                                             deviation                                                                           Electrode                                           property                                                                            strip width                                                                           *.sup.3 Frictional                                                                  along position                                No.         (g/m.sup.2)                                                                         (g/m.sup.2)                                                                           coefficient                                                                         strip width                                                                         property                                                                            Remarks                           __________________________________________________________________________    Invention's Example 7                                                                     4.9   0.26    0.138 0.003 --                                      Invention's Example 8                                                                     5.4   0.28    0.136 0.002 --                                      Invention's Example 9                                                                     5.7   0.27    0.136 0.002 --                                      Comparative Example 6                                                                     10.7  1.17    0.132 0.008 --    Uneven firing formed portions                                                 having                                                                        thick Γ phases.             Comparative Example 7                                                                     8.1   0.88    0.142 0.009 --    Uneven firing formed portions                                                 having                                                                        residual ζ phase.            Comparative Example 8                                                                     6.8   0.80    0.145 0.012 --    Uneven firing formed                                                          portions                                                                      having residual ζ                                                        phase.                            Comparative Example 9                                                                     4.2   0.25    0.138 0.004 x     Because of less amount of the                                                 top coat-                                                                     ing, low electrodeposition                                                    property is formed.               Invention's Example 10                                                                    4.5   0.27    0.122 0.002 ∘                           Invention's Example 11                                                                    4.7   0.28    0.121 0.002 ∘                           Invention's Example 12                                                                    3.8   0.22    0.122 0.002 ∘                           Invention's Example 13                                                                    5.2   0.25    0.123 0.002 ∘                           Comparative Example                                                                       8.9   0.65    0.123 0.003 ∘                                                                       Because of the strip leaving      10                                          temperature of high frequency                                                 induction                                                                     heating farnace is high,                                                      anti-powdering                                                                property is                       __________________________________________________________________________                                                low.                               *.sup.1 Good if it is not more than 6 g/m.sup.2 (at a coating weight of 6     m.sup.2)                                                                      *.sup.2 Good if it is not more than 0.3 g/m.sup.2                             *.sup.3 Good if it is not more than 0.14 without top coating                  0.13 with top coating                                                         *.sup.4 Good if it is not more than 0.003                                

What is claimed is:
 1. A process for manufacturing galvannealed steelsheets having high press-formability and anti-powdering property,comprising:providing a zinc plating bath having an aluminum content ofat least 0.05 %, but less than 0.13 %, the balance of the bathcomposition being zinc and unavoidable impurities, said bath having atemperature not exceeding 460° C.; prior to immersing a steel strip insaid bath, setting the temperature of said steel strip by heat treatmentsatisfying the following relationship:

    437.5×[Al%]+428>T≧437.5×[Al%]+408

where [A1%]: the aluminum content (%) of said bath; and T: thetemperature (°C.) of said strip entering said bath, so that any reactioncausing the alloying of iron and zinc may be prevented from occurring insaid bath; plating in said zinc plating bath said steel strip which wassubjected to said heat treatment; controlling the coating weight of aplating layer; and subjecting said coated strip to alloying treatment ina high-frequency induction furnace so that its coating may have an ironcontent of 8 to 12%, by heating said coated strip in said furnace sothat said coated strip has a temperature of from over 495° C. to 520° C.when leaving said furnace, and cooling said coated strip.
 2. A processfor manufacturing galvannealed steel sheets according to claim 1,further comprising plating said coated strip with a top coating havingan iron content of at least 50 % and a coating weight of at least 2g/m².